Oil sand process streams comprise bitumen (a heavy oil), water, and mineral solids of various particle sizes. As used herein, “oil sands process stream” means any stream produced during the extraction of bitumen from oil sands, including tailings streams and streams produced during the treatment of tailings prior to reclamation. The composition of such streams can have a strong impact on the recovery of bitumen during the hot water extraction process of oil sand slurry. For example, it is well known that high concentrations of fine particles can have a negative impact on the recovery of bitumen, and that the dosage of process aids, such as caustic, can be adjusted to help mitigate poor bitumen recovery if the amount of fine particles can be determined. The tailings streams from the extraction process segregate in settling basins, forming clarified process water for reuse, sand beaches, and a material high in water and fine solids content known as fluid fine tailings (FFT). When tailings materials are mixed with gypsum and sand to produce a non-segregating mixture (i.e. composite tailings) or mixed with polymer flocculants and gypsum and then centrifuged to produce a stronger cake material (i.e. centrifuged tailings), knowing the composition of the feed and product streams, including the concentration of fine particles, is important to produce materials that meet the required quality specifications and to optimize the dosage of process aids such as flocculant and gypsum.
The Dean-Stark extraction method is currently the industry reference method for determining the concentrations of bitumen, water, and/or solids of an oil sand process sample. A weighed sample is separated into bitumen, water, and solids by refluxing toluene in a Soxhlet extraction apparatus. Condensed toluene and co-distilled water are continuously separated in a trap designed to recycle the solvent through the extraction thimble, dissolving the bitumen present in the sample, while the water is retained in the trap. Full extraction of bitumen from the solids can often take hours to complete. Once the three components have been physically separated, they can be determined by various means.
Given the long analysis time of Dean-Stark extraction, faster methods are often used to monitor oil sand process streams. These fast methods typically sacrifice some accuracy and/or repeatability in order to achieve a shorter analysis time. They may rely on an extraction step, centrifugation step, filtration step, and/or drying step to separate various components prior to measurement, which adds to the total analysis time. Process conditions can quickly change within minutes, making ever shorter analysis times desirable.
Before particle size analysis of the mineral solids within oil sand process streams can be performed, the solids typically need to be cleaned and dried to remove the bitumen and water (e.g. by Dean-Stark extraction or repeated cold solvent washing and centrifugation followed by drying). The clean and dry solids then need to be disaggregated, dispersed in a suitable aqueous solution that results in a stable, fully dispersed suspension of fine particles, then measured by a suitable particle sizing analysis technique such as laser diffraction or wet sieve. Failure to remove the bitumen can lead to erroneous results, such as including the bitumen as part of the coarse solids, and can cause fouling issues with the particle sizing equipment.
These cleaning and measurement steps can take many hours or even days to perform to completion. It is therefore desirable to develop a faster method for quickly measuring the amount of bitumen, water, solids, and fine particles in oil sand process samples with relatively good accuracy (as compared to reference analysis methods), good precision, and within as short an analysis time as possible.
U.S. Pat. No. 8,547,096 discloses a method of quickly determining the composition of a sample including bitumen and water using low-field NMR. However, no information on the amount of fine particles is available. Also, calculating the solids content by subtracting the % bitumen and % water from 100% is not sufficiently accurate for some applications, for example, when measuring the % solids in centrifuge centrate samples when the % solids are routinely less than 5%.
U.S. Pat. No. 7,417,426 discloses a compact and portable NMR device for making a variety of NMR measurements on generic dispersions, with predictions of particle sizes and solid/liquid ratios in dispersions as some of the potential measurements that could be made. Their described methods of using a pure liquid and known samples to measure relaxation rate information to predict solid/liquid ratios, surface area, and particle size can be expected to work for simple dispersions of a single type of particle in a pure liquid. In this case, no information is provided on how to resolve the problem of overlapping signals from multiple fluids (e.g. bitumen and water). Specifically, no information is provided on how to deal with the issue of measuring overlapping signals of water-associated with fine particles and bitumen, both of which can have relaxation times on the order of milliseconds. Also, U.S. Pat. No. 7,417,426 demonstrates how the type of particle in the dispersion can have a strong impact on the measured relaxation rate for a given surface area. This complicates the prediction of particle size if there are multiple types of mineral particles (e.g. quartz, various clays, and other minerals), especially when one cannot assume that the mineral composition is consistent between different unknown samples.
U.S. Pat. No. 8,653,815 discloses a method for determining the particle size distribution of a subsurface rock formation using the relaxation data collected by a downhole NMR logging tool. This method is specific to downhole rock formations and also requires an elaborate correction procedure and potentially additional acoustic compressive strength measurements to be performed in order to achieve a certain level of agreement with reference particle sizing methods. No information is provided on how to deal with the issue of measuring overlapping signals of water-associated with fine particles and a heavy oil such as bitumen.
U.S. Patent Application No. 2014/0132259 discloses a method for determining particle size distribution of a subsurface rock formation using the relaxation data collected by a downhole NMR logging tool when there are at least two fluids present. In this case, other measurements such as electrical resistivity are needed to help determine fractional fluid volumes. Also, samples of each fluid must be collected to measure their proprieties individually at the temperature and pressure of the downhole conditions. Further, assumptions must be made as to which fluid is present in pores below a cutoff size as part of the analysis. These subsurface rock formation measurement tools are not applicable to oil sand process streams where the compositions of the samples can vary significantly from very fluid (e.g. <5% solids) to very dense (e.g. 50-80% solids).
Thus, there is a need in the industry to be able to quickly measure the composition of various oil sand process streams, in particular, the solids content and particle size of the solids to help ensure reliable operations.